Thixotropic cementing compositions

ABSTRACT

A thixotropic cementing composition for use in oil and gas wells comprises a hydraulic cement slurry including as an additive a complex reaction product of a water soluble carboxyalkyl, hydroxyalkyl or mixed carboxyalkyl hydroxyalkyl ether of cellulose, and a polyvalent metal salt, for example a reaction product of hydroxyethylcellulose and zirconyl chloride.

This is a divisional application of application Ser. No. 242,738, filedApr. 10, 1972.

BACKGROUND OF THE INVENTION

The present invention relates to oil and gas well cements and to a noveladditive for imparting thixotropic properties thereto. By a thixotropiccement is meant a cement slurry which will develop a low viscosity inturbulent flow, but which exhibits a high viscosity on setting, thetransition being reversible.

The use of a thixotropic cement in connection with well treatments isknown in the prior art. A cement of this type is disclosed, for example,in U.S. Pat. No. 2,890,752, in which a nonaqueous cement slurry isformed by admixing with portland cement a liquid hydrocarbon base, suchas kerosene or diesel oil, with a sufficient amount of an alkali metalsoap to produce a thixotropic gel. However, compositions of thischaracter were designed to control the action of the water present inthe well on the cement, rather than the qualities of the cement slurryitself.

Accordingly, a need has existed for a cement slurry which will bepumpable as a thin or low viscosity body, but which when the slurrybecomes quiescent attains a relatively high viscosity, but without anysetting of the cement. A cement having this quality can be used as asqueeze cement for cementing a casing in an oil or gas well, or fordisplacing drilling mud from a well, and the like, without danger ofpremature total setting of the cement content if the pumping operationis interrupted. Resumption of pumping operations breaks the thixotropicgel formed and causes the viscosity to diminish to the lower valueswhich prevailed during the initial thin consistency pumping period.

GENERAL DESCRIPTION OF THE INVENTION

The present invention provides a novel thixotropic cementing compositionfor oil and gas wells and the like which when formed into a slurry andagitated, as by pumping, will achieve a low viscosity, e.g., 5 to 30poises, but which will achieve a relatively high viscosity, e.g., 50 to200 or more poises, but without premature setting of the cement, whenthe agitation is interrupted and the slurry becomes quiescent. Whenagitation or pumping is resumed, the viscosity of the slurry rapidlydecreases to a value sufficiently low to render the slurry pumpableagain. This sequence can be repeated several times or until the cementphase of the slurry hydrates and sets.

The present invention further provides a novel additive which impartsthixotropic properties to a cement slurry. This additive is a complexformed by the reaction of a salt of a polyvalent metal selected from thegroup consisting of zirconium, lead, chromium, ferric iron, hafnium,lanthanum, and yttrium, with a water soluble cellulose ether capable offorming said complex. The cellulose ether may be of the hydroxyalkyl,carboxyalkyl or carboxyalkylhydroxyalkyl variety.

The preferred polyvalent metal salt is zirconyl chloride (zirconiumoxychloride) and the practice of the invention will be illustrated withregard to the use of this compound, but it will be understood that thisselection is primarily for purposes of illustration and that theinvention is not to be regarded as limited thereto. Other examples ofpolyvalent metal salts which may be reacted with the cellulose ethers toform the additive of the invention include chronium nitrate, leadchromate, ferric chloride, and lead zirconate.

The water soluble ethers of cellulose which may be employed to form theadditive of the invention include hydroxyalkyl cellulose ethers, e.g.,hydroxyethyl cellulose; carboxyalkyl cellulose ethers, e.g.,carboxymethyl cellulose; and mixed ethers such as carboxyalkylhydroxyalkyl cellulose, e.g., carboxymethyl hydroxyethyl cellulose.These products, which are usually in the form of their sodium or otheralkali metal salts, are free-flowing white powders which must becompatible with the cement and must possess a sufficient degree ofpolymerization to act as viscosity-building agents for the cementslurry. They must also possess a molecular weight which will impart thedesired viscosity to the mix. The average number of hydroxyalkyl and/orcarboxyalkyl groups that are substituted per anhydroglucose unit of thecellulose is known as the "degree of substitution" or "D.S." The gradeswhich are useful for the purpose of the present invention generally havea D.S. between about 0.1 and 2.5, preferably between 0.3 and 1.5. Forinstance, sodium carboxymethyl cellulose ("CMC") having a D.S. from 0.7to 1.2 is a polyelectrolite which is ionic in solution and forms clearwater solutions over a pH range from 4 to 12 from which it isprecipitated only by acidification to pH 1 to 3. Hydroxyethyl cellulose,on the other hand, is nonionic in solution and may be preferred wherehigh tolerance for multivalent cations is desired.

These water soluble hydroxyalkyl cellulose ethers are prepared by knownmethods, by reacting alkali cellulose with an olefin oxide, such asethylene oxide, whereby a range of degree of substitution results whichdepends upon the amount of alkylating agent employed. The degree ofsubstitution influences the solubility properties of the hydroxyalkylcellulose ether. Suitable hydroxyethyl cellulose ethers are availablecommercially, for instance, under the designation "Natrosol 250"(Hercules, Inc.) in different grades which exhibit various viscositiesin aqueous solution, as measured at concentrations of 2% or 5%. Thus,for example, Natrosol M Grade hydroxyethyl cellulose has a Brookfieldviscosity of 4500 to 6500 in 2% solution at 25° C.

An example of water soluble mixed ether is carboxymethyl hydroxyethylcellulose, also customarily in the form of its sodium salt. A suitableform of this material is commercially available from Hercules, Inc.under the trade name "CMHEC-37M"; its average D.S. is approximately 0.3carboxymethyl and 0.7 hydroxyethyl, and it has a Brookfield viscosity inthe range from 200 to 1000 centipoises in 2% aqueous concentration at25° C. A different commercial grade of carboxymethyl hydroxyethylcellulose, which is known under the name "CMHEC-43L", has an averagesubstitution of 0.4 carboxymethyl and 0.3 hydroxyethyl and a Brookfieldviscosity in the 20-100 centipoise range in 2% concentration at 25° C.

Each of the foregoing types of water soluble alkyl ethers of celluloseshould possess similar degrees of substitution and polymerization. Thus,an appropriate range of molecular weight will lie between about 500,000and about 3,000,000. A lower molecular weight would not providesufficient viscosity, while a higher molecular weight would increase theviscosity to an undesirable degree.

The presently preferred additive according to the invention is a complexof zirconyl chloride and hydroxyethylcellulose, and the practice of theinvention will be illustrated with regard thereto.

As far as the hydraulic cement is concerned, this may include anyportland, aluminous, or pozzolanic hydraulic cement, portland cementbeing preferred.

A small amount of bentonite, e.g., from about 1% to 3% by weight mayalso be included as a thickener.

The addition of hydroxyalkyl cellulose ethers to cement slurries forwell cementing purposes is disclosed in U.S. Pat. No. 3,483,007, but nopolyvalent metal salts are present, and there is no indication thatthixotropic properties are imparted to the cement slurry. In U.S. Pat.No. 3,378,070 there is described a tough rubber-like complex ofhydroxyethylcellulose and a polyvalent metal ion for forming a viscoustough plug in an underground formation, but no cement is present, andthixotropic properties are not involved.

The water used in the present invention may be any fresh water,preferably water which is not contaminated with calcium or carbonates.Calcium salts, such as calcium chloride, have been found to destroy thestability of some foaming agents which may be optionally included toentrap air in the cement. Carbonates have been found to causeunpredictable flash setting of the cement, which is undesirable.

The water used in the present invention may contain other chloridesalts, such as sodium chloride and potassium chloride, in amounts up toabout 10% by weight of the water without adversely affecting any of theproperties of the light weight cement.

Water should be present in the composition of the present invention inconcentrations of from about 23% to about 60% by weight of dry cement.Water concentrations of less than about 23% by weight of cement havebeen found to result in a cement having unacceptably low strengths. Thisis thought to be because at a water concentration of about 23% by weightof cement, there is not enough water to wet all the particles of thecement.

Water concentrations higher than about 60% by weight of cement have beenfound to greatly reduce the strength of the set cement and are thus notnormally useful in the present invention.

The preferred water concentration is about 30% by weight of cement. Thiswater concentration has been found to give the highest strengths in theset cement.

The composition of the present invention may also optionally includevarious auxiliaries or additives, such as so-called densifiers orfriction reducing agents or dispersing agents, or a combination of bothto improve the properties of the mix, as is otherwise well known in theart. The densifier, which may be a polymer such as, for example,polyvinyl pyrrolidone, serves to promote the retention of water in themix, while the dispersing agent improves flowability and assists thewater in wetting the cement particles. This makes it possible to usesmaller proportions of water to form the cement slurry, therebyproviding increased cementitious strength after the composition has set.A suitable type of combined densifier-dispersing agent is that disclosedin U.S. Pat. No. 3,359,225, issued Dec. 19, 1967, which is availablecommercially under the designation CFR-2, and comprises a combination ofpolyvinyl pyrrolidone and the sodium salt of a naphthalene sulfonatecondensed with formaldehyde. Other useful preparations of this type arethose sold under the trade names Lomar D, Tamol SM and TIC.

When such a densifier additive is used, it may be present in thecomposition in concentrations of from about 0.5% to about 2% by weightof cement. Concentrations of less than about 0.5% by weight of cementhave been found to have no significant dispersing effect. Concentrationsof greater than about 2% by weight of cement have been found to have noincreased dispersing effect above that provided by 2%.

The preferred concentration of densifier is about 0.75% by weight ofcement. Such a concentration of densifier makes possible a waterconcentration of about 30% by weight of cement. This water concentrationhas been found to provide the highest cementitious strengths after thecomposition has set.

In order to prepare a thixotropic cementing slurry in accordance withthe invention, a predetermined amount of water is agitated while thereis introduced a dry blend of the cement, the hydroxyalkylcellulose andthe polyvalent metal salt. The agitation is continued until the slurryforms a homogeneous mixture. Thus, a slurry containing the additiveprovided by reacting zirconyl chloride and "Natrosol 250"hydroxyethylcellulose has an initial viscosity of the order of about 17poises. The slurry is ready to be pumped into the well, and this can bedone with little change in viscosity. When the period of pumping isinterrupted the viscosity may rise to a level of about 75 to 100 poiseswithin about 20 minutes. When pumping is resumed, the viscosity rapidlydecreases to render the slurry pumpable again. The water solublecellulose derivative has no adverse effect on the cement.

The percentage of hydroxyalkyl or carboxyalkyl cellulose is not criticaland may vary from about 0.10% to about 0.75% by weight of the drycement, the optimum concentration in any given case being dependent uponthe type of cement and the grade of cellulose ether derivative used.

The percentage of metal salt employed will depend upon the type ofcement, the type of cellulose ether, and upon well conditions, but willordinarily be in the range of about 0.10% and about 3.0% by weight ofthe dry cement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples will serve to illustrate the practice of theinvention, but are not to be regarded as limiting:

EXAMPLE 1

A dry blend is prepared by admixing 100 parts by weight of portlandcement (API Class H) with 0.25 parts by weight of "Natrosol 250"hydroxyethyl cellulose and 2.0 parts by weight of zirconyl chloride(zirconium oxychloride). The dry mixture is added to 46 parts of waterwith vigorous agitation, and the agitation is continued until ahomogeneous slurry is obtained. The initial viscosity of this slurry is17 poises. The slurry is pumped into a simulated well for 10 minuteswith substantially no change in viscosity. When the pumping isinterrupted, the viscosity rises to 75-100 poises within a period of 20minutes, but upon resumption of pumping the viscosity rapidly decreasesagain so that the slurry becomes pumpable once more. This sequence isrepeated several times until the cement phase of the slurry hydrates andsets. Total setting time is about 2 hours.

EXAMPLE 2

Proceeding as in Example 1, analogous tests were made using 2 parts byweight of each of chromium nitrate, lead chromate, and ferric chloride.The results were essentially similar.

The scope of the present invention is particularly pointed out in theappended claims.

What is claimed is:
 1. A thixotropic slurry for cementing oil and gaswells and the like, comprising portland cement and a complex formed bythe reaction of zirconyl chloride with hydroxyethyl cellulose, theslurry being characterized by having a viscosity in the range from about5 to 30 poises when agitated and in the range from 50 to 200 poises whenquiescent.
 2. The composition of claim 1 which further includes adensifier and dispersing agent.
 3. A dry mixture of a hydraulic cementwith from about 0.1% to about 0.75% of hydroxyethyl cellulose and fromabout 0.1% to about 3.0% of zirconyl chloride, the percentages being byweight of the cement.
 4. A thixotropic cement slurry having a relativelyhigh viscosity under quiescent conditions and low viscosity underturbulent conditions, comprising a mixture of water, hydraulic cementand a thixotropic agent formed by complexing a polyvalent metal saltselected from the group consisting of zirconium, lead, chromium, ferriciron, hafnium, lanthanum and yttrium and a water soluble cellulose etherselected from the group consisting of hydroxyethyl cellulose,carboxymethyl cellulose, carboxyethyl cellulose, hydroxyethylcarboxymethyl cellulose or combinations thereof.
 5. A thixotropic cementslurry of claim 4 having a relatively high viscosity under quiescentconditions and low viscosity under turbulent conditions, comprising amixture of water, hydraulic cement and a thixotropic agent formed bycomplexing in the presence of cement, a polyvalent metal salt and awater soluble cellulose ether.
 6. A thixotropic cement slurry of claim 5wherein the polyvalent metal salt is a salt of zirconium.
 7. Athixotropic cement slurry of claim 5 wherein the polyvalent metal saltis a salt of lead.
 8. A thixotropic cement slurry of claim 5 wherein thepolyvalent metal salt is a salt of chromium.
 9. A thixotropic cementslurry of claim 5 wherein the polyvalent metal salt is a salt of ferriciron.
 10. A thixotropic cement slurry of claim 5 wherein the polyvalentmetal salt is a salt of hafnium.
 11. A thixotropic cement slurry ofclaim 5 wherein the polyvalent metal salt is a salt of lanthanum.
 12. Athixotropic cement slurry of claim 5 wherein the polyvalent metal saltis a salt of yttrium.
 13. A thixotropic cement slurry of claim 6 inwhich the soluble cellulose ether is carboxymethyl cellulose,carboxyethyl cellulose or combinations thereof.
 14. A thixotropic cementslurry of claim 7 in which the soluble cellulose ether is carboxymethylcellulose, carboxyethyl cellulose or combinations thereof.
 15. Athixotropic cement slurry of claim 8 in which the soluble celluloseether is carboxymethyl cellulose, carboxyethyl cellulose or combinationsthereof.
 16. A thixotropic cement slurry of claim 9 wherein the solublecellulose ether is carboxymethyl cellulose, carboxyethyl cellulose orcombinations thereof.
 17. A thixotropic cement slurry of claim 10wherein the soluble cellulose ether is carboxymethyl cellulose,carboxyethyl cellulose or combinations thereof.
 18. A thixotropic cementslurry of claim 11 wherein the soluble cellulose ether is carboxymethylcellulose, carboxyethyl cellulose or combinations thereof.
 19. Athixotropic cement slurry of claim 12 wherein the soluble celluloseether is carboxymethyl cellulose, carboxyethyl cellulose orcombinations.
 20. An additive which provides thixotropic properties ofrelatively high viscosity under quiescent conditions and low viscosityunder turbulent conditions comprising a complex of a polyvalent metalselected from zirconium, lead, chromium, ferric iron, hafnium, lanthanumand yttrium complexed under alkaline cement conditions with a watersoluble cellulose ether selected from hydroxyalkyl cellulose,carboxyalkyl cellulose ethers, mixed ethers and combinations thereof.21. An additive of claim 20 wherein the polyvalent metal is zirconiumand the water soluble ether comprises hydroxyethyl cellulose ether. 22.An additive of claim 20 wherein the polyvalent metal is zirconium andthe water soluble ether comprises carboxymethyl cellulose, carboxyethylcellulose or mixtures thereof.
 23. An additive of claim 20 wherein thepolyvalent metal is zirconium, lead, chromium, ferric iron, hafnium,lanthanum or yttrium and the water soluble ether is hydroxyethylcellulose, carboxymethyl cellulose, carboxyethyl cellulose, hydroxyethylcarboxymethyl cellulose or combinations thereof.
 24. A thixotropiccement slurry having a relatively high viscosity under quiescentconditions and low viscosity under turbulent conditions, comprising amixture of water, hydraulic cement and a thixotropic agent formed bycomplexing a polyvalent metal salt selected from the group consisting ofzirconium, lead, chromium, ferric iron, hafnium, lanthanum and yttriumand a water soluble hydroxyethyl cellulose.
 25. A thixotropic cementslurry having a relatively high viscosity under quiescent conditions andlow viscosity under turbulent conditions comprising a mixture of water,hydraulic cement and a thixotropic agent formed by complexing zirconylchloride and a water soluble cellulose ether selected from the groupconsisting of hydroxyethyl cellulose, carboxymethyl cellulose,carboxyethyl cellulose, hydroxyethyl carboxymethyl cellulose orcombinations thereof.